The overall specific aim of this application is to determine the mechanism of action of dialyzable transfer factor. The experiments will be conducted in mice. The first aim is to test the hypothesis that transfer factor functions through antigen-specific mechanisms. A component of these studies will be characterization of the effects of transfer factor in immune responses in vitro by studying production of lymphokines and T-cell proliferation and relating these results to in vivo responses. The second specific aim is directed toward testing possible modes of action by studying the effects of transfer factor on immune responses in nude mice in tolerant animals and in genetically-restricted responses. The possibility that transfer factor activates helper function will be tested. Studies of possible recognition mechanisms will assess direct interactions between transfer factor and antigens and the effect of antibodies to la determinants. The third specific aim is to isolate and purify specific transfer factors. These studies should define the nature of the apparent antigen-specificity of transfer factor and provide evidence in favor of certain mechanisms of action while excluding other possible mechanisms.